Hybrid powertrains include an internal combustion engine and one or more electric machines. A controller is programmed to automatically vary use of the engine and the electric machine(s) during different vehicle conditions in order to select an optimal powertrain operating mode. Some hybrid powertrain designs use dual electric motor-based systems for engine start/stop functionality and onboard power regeneration. For example, a conventional 12 VDC auxiliary starter motor may be used to provide geared engine start/stop control while use of a high-voltage or an auxiliary voltage belted alternator-starter (BAS) system enables one of the electric machines to assist engine torque or recharge one or more batteries.